Process of producing nitrogen rich wrought austenitic alloys



United States Patent PROCESS OF PRODUCING NITROGEN RICH WROUGHT AUSTENITIC ALLOYS Eric R. Morgan, lnkster, and Victor F. Zackay, Detroit, Micln, assignors to Ford Motor Company, Dearborn, Micln, a corporation of Delaware No Drawing. Application February ,15, 1955, Serial No. 488,409

2 Claims. (Cl. 148--3) This invention is concerned with a process for the manufacture of sound nitrogen rich austenitic alloys which are fully and stably austenitic and which require the utilization of greatly reduced amounts of nickel and in some situations permit the total elimination of this element.

The prior art has long been acquainted with the addition of substantial quantities of nitrogen to iron alloys to take advantage of the very potent austenitizing influence of this element and its insignificant cost. This substitution has effected substantial economies in the use of nickel which is notoriously expensive and at times difficut to obtain. Despite the economic and technical advantages attendant upon this substitution little commercial use has been made of nitrogen due to the difliculties encountered in producing ingots having the desired content of this fugitive element.

No difiiculty is encountered in introducing almost any desired nitrogen content into the molten bath. However, the solubility of nitrogen in austenite which has just been rejected from the liquid is very much less than its solubility in the molten bath. This results in an ebullition of nitrogen from the freezing alloy and the consequent production of an ingot in which the nitrogen content is lower than that desired to take full advantage of its potentialities. Recourse has been had to the use of superatmospheric pressure ove the metal during pouring and solidification, but this complicates the process and reduces the size of ingot easily handled to a point where the process is not economically attractive except under unusal circumstances.

This invention is predicated upon the discovery that this usually unwanted precipitation or ebullition of nitrogen may be permitted to occur, the nitrogen so freed trapped in the solidifying ingot and later redissolved and a sound ingot produced. This is possible because the solubility of nitrogen in solid austenite rises as the metal cools from the point of solidification. Nitrogen is much more soluble at temperatures at which metal of this type is usually hnt worked than it is at the freezing point of the metal.

The exact chemical composition of the metal upon which this invention is operative is not critical but should fall within the following tolerances:

Percent Chromium to 30 Manganese Sto 20 Nitrogen 0.3 to 1.0 Carbon 0.0 to 0.4 Molybdenum 0.0 to 3.0 Phosphorous 0.0 to 0.2 Tungsten 0.0to 6.0 Silicon 0.0m 3.0 Nickel -i 0.0 to 10.0

Vanadium 0.0to 0.5

While the exact composition of this alloy is not critical,

' the austenitizing elements such as carbon, nitrogen, manwhich tend to render nitrogen insoluble in austenite. The

principal element in this respect is silicon although nickel,

. and to a much lesser extend, molybdenum, tungsten, and

phosphorous also tend to render nitrogen less soluble in austenite. Thus, a balance must be struck between the benefits or advantages to be gained from the use of these elements and their tendency to render nitrogen less soluble in the alloy. As is well understood in the art nitrogen may be added as gaseous nitrogen, as a cyanide, as a cyanamide or as nitrides of appropriate metals such as vanadium, chromium or manganese.

In the execution of this invention a melt of the desired composition is prepared which contains at least as much nitrogen as is desired in the completed alloy and preferably contains a considerable excess of this gas. This molten nitrogenous alloy is poured into an ingot mold and solidification permitted to initiate. This solidification will result in the rejection and ebullition of a portion of the nitrogen from the metal due to the decrease in. solubility of nitrogen as the metal passes from the liquid to the solid phase. When the solidifying ingot has acquired a solid skin or shell of the desired thickness, it is capped preferably by placing a comparatively cold slab of metal across the upper surface of the ingot to freeze the adjacent metal and trap the nitrogen remaining in the ingot within it. After this capping operation the rejection of nitrogen continues as the ingot solidifies and the nitrogen so liberated is trapped in the form of gas filled bubbles or pockets in the ingot. Ordinarily an ingot containing much entrapped gas is considered worthless and unsound.

However, a gassy ingot produced according to the instant invention is hot worked in the ordinary range of temperatures and the combination of heating and hot working causes the resolution of the nitrogen is the austenite and hence makes available all of the desirable properties of this element. This hot working is done at a temperature which is favorable to the solution of nitrogen in usual mill operation. "I he hot working operation tends to deform each individual gas pocket and make available less space for occupancy by the same volume of gas. This results in a tremendous rise in gas pressure which expedites its solution in the solid metal. The net result of this hot rolling is that the nitrogen entrapped in the ingot is substantially all redissolved in the solid metal and at the same time the sides of the nitrogen pockets weld together to produce a sound and gas free ingot. The yield from any given ingot is improved because the evolving nitrogen tends to reduce the natural shrinkage of the metal on solidification so that the pipe produced in the ingot is smaller and less destructive. This makes possible an increased-yield from each ingot. This increased yield has an important economic advantage in enabling this steel to compete with conventional austenitic steels.

The following tabulation is presented as illustrative of alloys prepared during the perfection of this invention.

Alloy Cr Mn Mo N 0 Ni. St P Remarks A B O D Number Norm-Column A expresses the creep rupture life in hours at a temperature of 1200F. and a stress or 40,000 pounds per square inch. Column 0 expresses the yield strength in pounds per square inch (0.2 inch oflset). Column in percent (one inch).

Attention is specifically invited to alloys 49 and 162. Although one of these alloys (162) as cast is gassy due to lower levels of nitrogen absorbing elements such as chromium and manganese and the other (49) is not, the originally gassy ingot becomes superior in elevated temperature physical properties after hot working. Within atempcraturc range of 1200 to 1400" F. these alloys exhibited strength comparable to the best of the iron base alloys such as 16-25-6 which is a standard high tempcrature alloy containing 16 percent chromium, 25 percent nickcl and 6 percent molybdenum.

The effect of higher levels of molybdenum, nickel, silicon and phosphorous on the gassing action of austcnitic steels is illustrated by alloys 69, 73 and 167. Although the nitrogen levels of these alloys are similar to that of alloy 49, all exhibited gas porosity on casting.

This invention has been particularly described with reference to austenitic alloys predicated upon iron, chromium and manganese alloys, but it is by no means so limited. It is equally applicable to any iron alloy which is stably austenitic at room temperatures and in which nitrogen is less soluble at the freezing temperature than at hot working temperatures.

What is claimed is:

1. The process of producinga sound, iron base, stably austenitic alloy containing as necessary elements from ten to thirty percent of chromium, five to twenty percent of manganese and from three tenths to one percent of nitrogen and in which the austenitizing elements and the ferrite forming elements are proportioned to permit a. stably austenitic alloy at the desired nitrogen level com- B expresses the tensile strength in pounds or square inch. Column expresses the elongation prising producing a melt of the desired composition. casting the melt into an ingot mold, permitting a solid skin to form on the ingot with the simultaneous cbullition of nitrogen, capping the ingot to entrap a substantial amount of nitrogen in the gaseous form as bubbles, permitting the ingot to completely solidify and subjecting the ingot to intensive hot working whereby the gaseous nitrogen contained in the ingot is redissolved and the sides of the nitrogen cavities welded shut to produce a sound and mechanically strong metal.

2. The process of producing a sound iron base, stably austcnitic alloy containing from three tenths of one percent to one percent nitrogen and in which the ferrite forming alloying elements and the austenite forming alloying elements are balanced to produce an alloy which is stably austenitic at room temperature and at the dcsired nitrogen level comprising establishing a melt of the desired composition, casting the melt into an ingot mold, permitting a solid skin to form on the ingot with the simultaneous ebullition of nitrogen, capping the ingot to entrap a substantial amount of nitrogen in the gaseous form as bubbles, permitting the ingot to completely solidify and subjecting the ingot to intensive hot working whereby the gaseous nitrogen contained in the ingot is redissolved and the sides of the nitrogen cavities welded shut to produce a sound and mechanically sound metal.

References Cited in the tile ofthis patent UNITED STATES PATENTS Belding Feb. 13, 1940 

1. TH EPROCESS OF PRODUCING A SOUND, IRON BASE, STABLY AUSTENIC ALLOY CONTAINING AS NECESSARY ELEMENTS FROM TEN TO THIRTY PERCENT OF CHRONIUM, FIVE TO TWENTY PERCENT OF MANGANESE AND FROM THREE TENTHS TO ONE PERCENT OF NITROGEN AND IN WHICH THE AUSENTENITIZING ELEMENTS AND THE FERRITE FORMING ELEMENTS ARE PROPORTIONED TO PERMIT A STABLY AUSTENTIC ALLOY AT THE DESIRED NITROGEN LEVEL COMPRISING PRODUCING A MELT OF THE DESIRED COMPOSITION, CASTING THE MELT INTO AN INGOT MOLD, PERMITTING A SOLID SKIN TO FORM ON TH INGOT WITH THE SIMULTANEOUS EBULLITION OF NITROGEN, CAPPING THE INGOT TO ENTRAP A SUBSTANTIAL AMOUNT OF NITROGEN IN THE GASEOUS FORM AS BUBBLES, PERMITTING THE INGOT TO COMPLETELY SOLIDIFY AND SUBJECTING THE INGOT TO INTENSIVE HOT WORKING WHEREBY THE GASEOUS NITROGEN CONTAINED IN THE INGOT IS REDISSOLVED AND THE SIDES OF THE NITROGEN CAVITIES WELDED SHUT TO PRODUCE A SOUND AND MECHANICALLY STRONG METAL. 